The ultrasonic inspection of materials is often performed using refracted beams of sonic energy having frequencies in the range from 1 to 10 MHz. The means of producing such beams are well known in the art of ultrasonic transducer design, and either longitudinal or shear wave beams can be produced.
The waveform and associated frequency spectrum of a nominal 2.25-MHz impulse response for a conventional transducer are shown in FIGS. 1 and 2, respectively. The waveform of FIG. 1 has a damped oscillatory nature and a spectral content spread over a significant band of frequencies in the bell-shaped distribution shown in FIG. 2. The bandwidth is determined by the damping built into the transducer design, and either highly damped or lowly damped designs are available. Thus, either broadband or narrowband transducers can be obtained, depending on the application requirements.
If the transducer, rather than being pulsed, is instead driven with a sinusoidal excitation of finite duration, the resulting waveform can be derived from the convolution of the impulse response with the exciting function. A typical "tone burst" is shown in FIG. 3 for a nominal 2.25-MHz transducer. As can be seen in FIG. 3, the front-end of the waveform has an appreciable rise-time. The frequency spectrum of this waveform (see FIG. 4) is considerably narrower than that shown in FIG. 2, but is still far from monochromatic (i.e., possessing only a very narrow range or a single frequency), when compared with the spectrum of a conventional monochromatic source, such as that shown in FIG. 5.
In most standard applications of ultrasonics, the bandwidth is not critical and may be within predetermined broad limits. But for the purpose of detecting the size of flaws in a component of a nuclear reactor by means of scattered ultrasonic waves, narrow bandwidth can be an important factor in achieving accuracy. This implies that a monochromatic source of ultrasonic energy having a spectrum similar to that depicted in FIG. 5 would be a valuable tool, even though the spectral peak would be reduced.